When you are winning, it's easier to keep winning, but when you start losing and then being able to regain the winning form, that's what makes a champion.

Fed became more of a champion / legend in 2009 after bouncing back from losing all those matches in 2008.
Same with Nadal, he too became a greater champion in 2010 after bouncing back from his 2009 season.

The mark of a champion is measured by how they bounce back from setbacks, which is why Borg should never be considered greater than Fed or Nadal, simply because he ran away from the big bad bully.